Species-Specific Allometric Equations for Predicting Belowground Root Biomass in Plantations: Case Study of Spotted Gums (Corymbia citriodora subspecies variegata) in Queensland

Spotted gum (Corymbia citriodora spp. variegata; CCV) has been widely planted, has a wide natural distribution, and is the most important commercially harvested hardwood species in Queensland, Australia. It has a great capacity to sequester carbon, thus reducing the impact of CO2 emissions on climate. Belowground root biomass (BGB) plays an important role as a carbon sink in terrestrial ecosystems. To explore the potential of biomass and carbon accumulation belowground, we developed and validated models for CCV plantations in Queensland. The roots of twenty-three individual trees (size range 11.8–42.0 cm diameter at breast height) from three sites were excavated to a 1-m depth and were weighed to obtain BGB. Weighted nonlinear regression models were most reliable for estimating BGB. To evaluate the candidate models, the data set was cross-validated with 70% of the data used for training and 30% of the data used for testing. The cross-validation process was repeated 23 times and the validation of the models were averaged over 23 iterations. The best model for predicting spotted gum BGB was based on a single parameter, with the diameter at breast height (D) as an independent variable. The best equation BGB = 0.02933 × D2.5805 had an adjusted R2 of 0.854 and a mean absolute percentage error of 0.090%. This equation was tested against published BGB equations; the findings from this are discussed. Our equation is recommended to allow improved estimates of BGB for this species

Species-Specific Allometric Equations for Predicting Belowground Root Biomass in Plantations: Case Study of Spotted Gums (Corymbia citriodora subspecies variegata) in Queensland

Spotted gum (Corymbia citriodora spp. variegata; CCV) has been widely planted, has a wide natural distribution, and is the most important commercially harvested hardwood species in Queensland, Australia. It has a great capacity to sequester carbon, thus reducing the impact of CO2 emissions on climate. Belowground root biomass (BGB) plays an important role as a carbon sink in terrestrial ecosystems. To explore the potential of biomass and carbon accumulation belowground, we developed and validated models for CCV plantations in Queensland. The roots of twenty-three individual trees (size range 11.8–42.0 cm diameter at breast height) from three sites were excavated to a 1-m depth and were weighed to obtain BGB. Weighted nonlinear regression models were most reliable for estimating BGB. To evaluate the candidate models, the data set was cross-validated with 70% of the data used for training and 30% of the data used for testing. The cross-validation process was repeated 23 times and the validation of the models were averaged over 23 iterations. The best model for predicting spotted gum BGB was based on a single parameter, with the diameter at breast height (D) as an independent variable. The best equation BGB = 0.02933 × D2.5805 had an adjusted R2 of 0.854 and a mean absolute percentage error of 0.090%. This equation was tested against published BGB equations; the findings from this are discussed. Our equation is recommended to allow improved estimates of BGB for this species

Improving Productivity in Mixed-Species Plantations

Mixed species plantations are often promoted as being environmentally preferable to monocultures, but are rarely considered operationally viable by commercial forest growers. Despite many publications documenting benefits demonstrated in research studies, and despite continuing calls from a wide range of advocates for mixed-species plantations, polyculture remains the exception rather than the rule in industrial plantation forestry. The following observations are drawn from a recent workshop: - innovative experiment designs and analytical techniques are available to examine species interactions; - despite the enthusiasm for polycultures, relatively few robust experiments have been established, and even fewer have been maintained long enough to allow rotation-length consequences to be evaluated; - commercial polyculture plantations are even more scarce than experiments, and rarely offer data to support publication of financial analyses; - small landholders appear to be the main innovators in establishing and demonstrating polyculture plantations. To provide the evidence to encourage industrial uptake of polyculture plantations, there is a need for - a co-ordinated series of long-term trials, well replicated in time and space, using a standardised design with several treatments (species composition) and comparable species; - operational-scale demonstration plantings that gather ecological, financial and social data as well as the conventional production data

Mixed Species Plantations: Prospects and Challenges

About 2% of English-language literature on plantations deals with mixed-species plantations, but only a tiny proportion

Growth of eucalyptus pellita in mixed species and monoculture plantations

Eucalyptus pellita is a commercially important plantation hardwood species for the humid tropics of north Queensland. This species is favoured by both small-scale growers for use in mixed species woodlots targeting low-volume high-value sawn timber, and also by industrial forest companies growing monocultures for integrated pulp – sawn timber regimes. This study investigated whether mixed-species designs can increase the growth of this tropical eucalypt when compared to monocultures. A replacement series experiment with monocultures of Eucalyptus pellita (E) and Acacia peregrina (A) and mixtures in various proportions (75E:25A, 50E:50A, 25E:75A) was used to examine questions about growth and productivity. The trial was located on the Atherton Tablelands of north Queensland, Australia. High mortality in the establishment phase due to repeated damage by tropical cyclones altered the trial design. Effects of experimental designs on tree growth were estimated using a linear mixed effects model with restricted maximum likelihood analysis (REML). Volume growth of individual eucalypt trees were positively affected by the presence of acacia trees at age five years and this effect generally increased with time up to age 10 years. However, the stand volume and basal area increased with increasing proportions of E. pellita, due to its larger individual tree size. Conventional analysis did not offer convincing support for mixed-species designs. Preliminary individual-based modelling using a modified Hegyi competition index offered a solution and an equation that indicates acacias have positive ecological interactions (facilitation or competitive reduction), and definitely do not cause competition like E. pellita. These results suggest that significantly increased growth rates could be achieved with mixed-species designs over E. pellita monocultures. This statistical methodology could enable a better 4 understanding of species interactions in similarly altered experiments, or undesigned mixed-species plantations. The effects of trees on soils are highly variable and highly site and species specific. That trees can change soil chemistry over time is well established. The soil chemical properties under the eucalypt: acacia experiment were compared to several potential baseline data sources: the reference description of this soil type; those measured at 7 months after planting; and with those of soils under two adjacent vegetation types (forest and pasture) when the experiment was aged 9 years. At 9 years after planting soil total nitrogen increased with increasing proportion of acacias in the treatment. The mean total N under the acacia monoculture was significantly higher (P = 0.041) than that of either the eucalypt monoculture, or the surrounding pasture. The proportion of acacia in the treatment was positively linearly correlated with soil total N (r2 = 0.46; P = 0.018). Soils under the eucalypt monocultures were more similar to those under pasture for a range of soil chemical properties, compared with soils under treatments containing acacias. Results from this site show that the two species alter the soil chemistry in different ways. It is possible that the increased total N under the acacias could be facilitating the growth of the E. pellita, however without n-fixation analysis or tissue sampling it is not possible to confirm that the eucalypt is using the N. Similar cause and effect (or ‘supply and use’) questions also remain for soil pH and available phosphorus changes with increasing acacia in treatment. This study also demonstrates the difficulty in monitoring changes in soil properties over long cycles of forest plantations. The photosynthetic response to light was assessed in the stratified canopy of the mixed species field trial of the eucalypt: acacia experiment, and among commonly planted taxa of E. pellita in glasshouse pot trials. In the field trial photosynthetic capacity of fully5 expanded sun and shade leaves of both species was measured. E. pellita has a wide natural distribution with considerable variation in morphology and growth within the species, with several provenances commonly planted in north Queensland. Photosynthetic capacity and leaf nutrient content of three of these taxa (two from northern occurrences and one from southern occurrences of E. pellita) were measured on two occasions in glasshouse pot trials. A non rectangular hyperbolic function was used to describe the light response curves, and analysis of variance was used to determine differences in the biologically relevant curve parameters between treatments. In the field trial sun and shade leaves of E. pellita produced similar light saturated photosynthetic rates, and experienced little competition for light from the acacia crowns. In contrast there was significant variation in the photosynthetic response between acacia sun and shade leaves. In the glasshouse trials, differences in leaf and petiole morphology were observed, which were coupled with differences in leaf nutrient content and highly significant variation in light saturated photosynthetic rate between the three taxa. This study characterised the light response of E. pellita and suggests that differences in physiological responses to resource availability should be expected among taxa within this species, which may be important for forest productivity models which endeavour to predict tree growth and resource use. An empirical model of growth of E. pellita from a designed monocultures vs. mixedspecies experiment has been used to explore system behaviour rather than predict production of this species from specific forests. This approach has allowed examination of the effect of plantation design on competition, soil nutrient pool change with time and physiological responses to light; leading to a greater understanding of why mixtures can lead to greater productivity than monocultures